The present invention relates to frequency shift keying (FSK) demodulating circuits and FM demodulation circuits and FM demodulation circuits.
It would be highly desirable to be able to build a complete radio frequency receiver without having to use conventional intermediate-frequency (IF) bandpass filters. Conventional IF filters are comparatively large in size as compared to the other components that are used in a receiver and typically the most expensive component used in the receiver. In addition, the technology of such filters does not lend itself to being incorporated on an integrated circuit. Therefore, the "single-chip" RF receivers currently being offered use external IF filters to function.
There has been interest in the past few years in so-called "zero IF" technology. With this technology, the local oscillator frequency injected into the mixer would be phase-locked to the transmitted signal's carrier frequency. This folds the sideband signals into a signal space from DC to the information cut-off frequency, thereby eliminating the need for conventional IF bandpass filters and allowing the design to use only low pass filters which can be incorporated into a monolithic integrated circuit. However, the approaches tried thus far have used a phased locked oscillator to lock the local-oscillator to the incoming carrier frequency and then to demodulate the signal by using a local oscillator frequency which has two outputs that are in phase quadrature with each other as mixer frequencies to demodulate an in-phase and a quadrature phase channel. The demodulator then discriminates between the folded sidebands created by each oscillator term by decoding the phase information in the demodulated sidebands.
FSK and FM receivers are commonly used in many applications. Primary uses of the present invention are for paging receivers and demodulation of conventional analog FM modulation.